In the Gulf of Mexico (GOM) (and in other locations around the world) millions of years ago, a thick layer of salt that can be thousands of feet deep was deposited by desiccation of sea water. Layers of sediment were deposited and compacted on top of the salt. Unlike sedimentary layers, salt behaves more like a fluid than a solid, changing its shape in response to surrounding sediment deposits and creating complicated fluid-like geometries. The salt present in the subsurface of the Gulf of Mexico has the appearance of solidified lava, with irregular structures of varying depth, as shown in FIG. 1. Also, unlike most sedimentary layers, which become denser with depth, salt maintains a nearly constant density at any depth. Accurate salt geometry definitions are needed for imaging sedimentary layers beneath salt.
Lately, with increasing speed and complexity of the computing resources, the imaging of geological structures under salt based on data acquired using seismic methods has become possible. Reverse-Time Migration (or RTM), which handles complex wave propagations in any direction without dip limitation, is now the standard imaging algorithm in seismic exploration. Conventional RTM salt model building is a top-down approach which consists of various steps of migration and interpretation: sediment-flood RTM and picking the top of salt (TOS), salt-flood RTM and picking the base of salt (BOS), and overhang RTM and interpretation as needed. However, in areas with complex salt geometry, such as a narrow mini-basin, the conventional approach may not be conducive to a correct salt body interpretation and can lead to poor images of formations under the salt.
Wave-fields undergo strong distortion when they propagate through the salt. Furthermore, in narrow mini-basins, the propagating wave-field becomes more complex due to multiple bounces at the sediment/salt interfaces. Ray-based migration cannot handle geological complexities, and one-way wave equation migration cannot properly handle turning and prismatic waves (i.e., waves that reflect at least twice before reaching the receiver). Although RTM provides a natural way to image prismatic waves, imaging salt surfaces around narrow mini-basin areas (i.e., deep “valleys” of sediment surrounded by the salt) can be quite difficult and ambiguous with conventional top-down salt model building. For example, FIG. 2 illustrates an image of formation layers underneath line A-A1 in FIG. 1, using conventional seismic data processing. The regions 210, 220, 230 and 240 in FIG. 2 emphasize fuzzy portions of the image due to ambiguous imaging of narrow mini-basins.
Therefore, more accurate methods for reconstructing shapes of salt based on seismic data are desirable.